The Boy's Playbook of Science Part 21

The year 1750 was remarkable for Volta's discoveries and Dr. Franklin's identification of the electricity of the machine with the stupendous effects of the thunderstorm. Sir Humphry Davy, in 1800, with his commanding genius, threw fresh light upon the already numerous electrical effects discovered. In 1821, Faraday commenced his studies in this branch of philosophy; which he has since so diligently followed up, that he has been for some years, and is still the first electrician of the age. From the commencement of the present century, discoveries have succeeded each other in regular order and with the most amazing results; and now electricity is regularly employed as a money-getting agent in the process of the electrotype and electro-silvering and gilding; also in the electric telegraph; and in a few years we may possibly see it commonly employed as a source of artificial light.

The nature of electricity, says Turner, like that of heat, is at present involved in obscurity. Both these principles, if really material, are so light, subtle, and diffuse, that it has. .h.i.therto been found impossible to recognise in them the ordinary characteristics of matter; and therefore electric phenomena may be referred, not to the agency of a specific substance, but to some property or state of common matter, just as sound and light are produced by a vibrating medium. But the effects of electricity are so similar to those of a mechanical agent, it appears so distinctly to emanate from substances which contain it in excess, and rends asunder all obstacles in its course so exactly like a body in rapid motion, that the impression of its existence as a distinct material substance _sui generis_ forces itself irresistibly on the mind.

All nations, accordingly, have spontaneously concurred in regarding electricity as a material principle; and scientific men give a preference to the same view, because it offers an easy explanation of phenomena, and suggests a natural language intelligible to all.

[Page 175]

There are five well-ascertained sources of electricity, and three which are considered to be uncertain. The five sources are friction, chemical action, heat, magnetism, peculiar animal organisms. The three uncertain sources are contact, evaporation, and the solar rays.

_First Experiment._

A stick of sealing-wax or a bit of gla.s.s tube, perfectly dry, rubbed against a warm piece of flannel, has elicited upon its surface a new power, which will attract bits of paper, straw, or other light materials; and after these substances are endowed with the same force, a repellent action takes place, and they fly off. One of the most convenient arrangements for making experiments with the attractive and repellent powers of electricity is to fix with sh.e.l.l-lac varnish round discs of gilt paper, of the size of a half-crown, at each end of a long straw that is supported about the centre with a silk thread, which may hang from the ceiling or any other convenient support. (Fig. 160.)

[Ill.u.s.tration: Fig. 160. A. The gla.s.s pillar support. B. Straw with discs, hanging by a silk thread.]

The varnish is easily prepared by placing four or eight ounces of sh.e.l.l-lac in a bottle, and pouring enough pyroxylic spirit (commonly termed wood naphtha) upon the lac to cover it. After a short time, and by agitation, solution takes place. In a variety of ways friction is proved to be a source of electricity, and forms a distinct branch of the science, under the name of _frictional_ electricity.

_Second Experiment._

The nature of chemical action has been already explained, and is alluded to here as a source of electricity of which the proof is very simple. A piece of copper and a similar-sized plate of zinc have attached to them copper wires; these plates are placed opposite to, but do not touch each other, in a vessel containing water acidulated with a small quant.i.ty of sulphuric acid. When the wires are brought in contact, a current of electricity circulates through the arrangement, but has no power to attract bits of paper, straw, &c. In order to ascertain whether the current of electricity pa.s.ses or not, a piece of covered copper wire is bent several times round a magnetic needle, so that it has freedom of motion inside the core or hollow formed by twisting the copper wire.

This arrangement, properly constructed, is called the galvanometer [Page 176] needle, and is invaluable as a means of ascertaining the pa.s.sage of electricity derived from chemical action. (Fig. 161.)

[Ill.u.s.tration: Fig. 161. A. The galvanometer needle. B. Vessel containing weak acid and the zinc and copper plates. The arrows show the path of the electric current.]

When the wires leading from the metal plates are connected with the extremities of the coil in the galvanometer, the needle is deflected or pushed aside to the right hand or to the left, according to the direction of the current.

_Third Experiment._

The third source of electricity is heat, and the effect of this agent is well shown by twisting together a piece of platinum and silver wire, so as to form one length. If the silver end is attached to any screw of the galvanometer, and the platinum end to the second screw, no movement of the magnetic needle takes place until the heat of a spirit-lamp is applied for a moment to the point of juncture between the silver and platinum wires, when the magnetic needle is immediately deflected.

[Ill.u.s.tration: Fig. 162. A. The galvanometer needle, with wires attached. S, S. Silver wire joined to P, P, the platinum wire. The heat of the spirit-lamp is applied at the point of juncture, +.]

_Fourth Experiment._

The fourth source of electricity--viz., magnetism--requires a somewhat more complicated arrangement; and a most delicate galvanometer needle must be provided, to which is attached the extremities of a long spiral coil of copper wire covered with cotton or silk. Every time a bar magnet is introduced inside the coil, so that the conducting wire cuts the magnetic curves, a deflection of the galvanometer needle takes place, [Page 177] and the same effect is produced on the withdrawal of the magnet, the needle being deflected in the opposite direction.

The magnetic spark can be obtained by employing a magnet of sufficient power; and the arrangement for this purpose is very simple. A cylinder of soft iron is provided, and round its centre are wound a few feet of covered thin copper wire, one end of which is terminated with a copper disc well amalgamated, and the other end, after being properly cleaned and coated with mercury, is brought into contact with the disc. Directly this cylinder is laid across the poles of the magnet, and as quickly removed, the point and disc, from the elasticity of the former, separate for the moment, the contact is broken between the point and disc, and a brilliant but tiny spark is apparent.

[Ill.u.s.tration: Fig. 163. A B. Horse-shoe magnet. C. Cylinder of soft iron. D. Coil of copper wire and contact breaker.]

_Fifth Experiment._

The fifth mode of procuring electricity would require the a.s.sistance of an electrical eel, a fine specimen of which (forty inches in length) was exhibited at the Adelaide Gallery some years ago. Various experiments were made with this animal, and the author had the pleasure of witnessing all the ordinary phenomena of frictional electricity, ill.u.s.trated by Dr. Faraday, with the a.s.sistance of the animal electricity derived from this curious creature. Recent experiments have, however, proved that the electric current is induced through the agency of the nervous [Page 178] system. This important fact has been communicated by M. Dubois-Raymond, whose experiment is thus recorded:--A cylinder of wood is firmly fixed against the edge of a table; two vessels filled with salt and water are placed on the table, in such a position that a person grasping the cylinder may, at the same time, insert the fore-finger of each hand in the water. Each vessel contains a metallic plate, and communicates, by two wires, with an extremely sensitive galvanometer. In the instrument employed by M. Dubois-Raymond, the wire is about 3 miles in length. The apparatus being thus arranged, the experimenter grasps the cylinder of wood firmly with both hands, at the same time dipping the fore-finger of each hand in the saline water.

The needle of the galvanometer remains undisturbed; the electric currents pa.s.sing by the nerves of each arm, and being of the same force, neutralize each other. Now, if the experimenter grasp with energy the cylinder of wood with the right hand, the left hand remaining relaxed and free, immediately the needle will move from west to south, and describe an angle of 30, 40, and even 50; on relaxing the grasp, the needle will return to its original position. The experiment may be reversed by employing the left arm, and leaving the right arm free: the needle will, in this case, be deflected from west to north. The reversing of the action of the needle proves the influence of the nervous force. The conditions, it may be added, essential to the success of the experiment are: 1st, Great muscular and nervous energy; 2nd, The contraction of only one arm at a time; 3rd, Dryness and cleanliness of skin; and 4th, Freedom from any kind of wound on the immersed part.

_Sixth Experiment._

In making electrical experiments of the simplest kind, it soon becomes apparent that certain substances, such as gla.s.s, sealing-wax, &c., retain the condition of electrical excitement; whilst other bodies, and especially the metals, seem wholly incapable of electrical excitation: hence the cla.s.sification of bodies into conductors and non-conductors of electricity. This arrangement is not strictly correct, because no substance can be regarded as absolutely a conductor, or _vice versa_. It is better to consider these terms as meaning the two extremes of a long chain of intermediate links, which pa.s.s by insensible gradations the one into the other. In the manufacture of electrical apparatus, gla.s.s is of course largely employed, and this substance, with bra.s.s and wood, const.i.tute the usual materials. One of the most instructive pieces of apparatus is the electroscope, which can be made with a gas jar, a cork, a piece of gla.s.s tube, bra.s.s wire and ball, or a flat disc of bra.s.s, with some Dutch metal, or still better, gold leaf. The latter is first cut into strips by retaining the leaf between a sheet of well-glazed paper and cutting through the paper and the copper or gold leaf, otherwise it would be impossible to cut the metal, on account of its excessive thinness, except with a gilder's knife and cushion. The cork is next fitted to the gas jar, and perforated with a hole to admit the gla.s.s tube, which must be thoroughly dry, and [Page 179] is best coated both inside and out with the sh.e.l.l-lac varnish described at page 175.

Some dry silk is wound round the bra.s.s wire, so that it remains fixed and upright in the gla.s.s tube, the end outside the jar having a ball, or still better, a flat disc of bra.s.s attached, and the other extremity being split so as to act like a pair of forceps, to retain a piece of card to which the gold leaves are attached. By removing the cork, tube, and bra.s.s wire bodily from the neck of the gas jar, and then in a perfectly still atmosphere carefully bringing the card, slightly wetted with gum at the extremity, on two of the cut gold leaves, they may be stuck on, and the whole is again arranged inside the dry gas jar, and forms the important instrument called the electroscope. (Fig. 164.) With the help of this arrangement, a number of highly instructive experiments are performed.

[Ill.u.s.tration: Fig. 164. A. The bra.s.s wire, with flat disc outside, and forceps holding gold leaf B inside the jar. C C. The gla.s.s tube.]

_Seventh Experiment._

First, the difference between conductors and non-conductors is admirably shown by rubbing a bit of sealing-wax against a piece of woollen cloth or flannel; on bringing the wax to the bra.s.s disc of the electroscope the gold leaves no longer hang quietly side by side, but stand out and repel each other, in obedience to the law "_that bodies similarly electrified repel each other_." If the bra.s.s cap is touched whilst the leaves are in this electrical state, they fall again to their original position, showing that sealing-wax, after being excited, retains its electrical condition, as also the gold leaves, because they are supported on gla.s.s, or what is termed _insulated_--_i.e._, cut off from conducting communication with surrounding objects. When, however, the sealing-wax is pa.s.sed through a damp hand, or the bra.s.s disc of the electroscope touched, the electricity is conveyed away to the earth, because the human body is a conductor of electricity.

_Eighth Experiment._

When a bra.s.s wire is rubbed and brought to the electroscope, the leaves do not move, in consequence of the electricity pa.s.sing away to the earth through the body as fast as it is generated: it is just like pouring water into a leaky cistern; but if the bra.s.s wire is tied to a long stick of sealing-wax, and this latter held in the hand whilst the wire is rubbed with a bit of flannel, then the gold leaves of the electroscope are affected, on account of the insulation of the metal, as every substance which can be rubbed (even fluids, as water) produces electricity.

[Page 180]

_Ninth Experiment._

An insulating stool is merely a piece of strong square board, supported on gla.s.s legs, which should be well varnished. If the a.s.sistant stands on this stool and touches the disc of the electroscope, no movement of the leaves takes place until his coat is briskly struck with a piece of dry silk or skin, when the usual repulsion occurs.

[Ill.u.s.tration: Fig. 165. a.s.sistant standing on the insulating stool and touching the disc of the electroscope whilst being struck with a dry handkerchief.]

_Tenth Experiment._

If a little powdered chalk is placed inside a pair of bellows, and then forcibly ejected on to the disc of the electroscope, the friction of the particles of chalk against the inside of the nozzle of the bellows and against the disc of the instrument soon liberates sufficient electricity to cause the gold leaves to stand out and repel each other.

_Eleventh Experiment._

Whilst the leaves of the electroscope are repelled from each other by the application of a bit of rubbed sealing-wax, they may be again caused to approach each other on bringing a dry gla.s.s tube previously rubbed with a silk-handkerchief; because the electricity obtained from sealing-wax is different from that procured from gla.s.s: the former is called _resinous_ or _negative_ electricity, the latter _positive_ or _vitreous_ electricity. Either, separately, is _repulsive_ of its own particles, but _attractive_ of the other. [Page 181] No electrical excitation can occur without the separation of these two curious states of electricity, and electrical quiescence takes place when the two electricities are brought together; hence the fall of the gold leaves repelled by rubbed wax when the excited gla.s.s is brought towards the disc of the electroscope. This experiment may be reversed by repelling the leaves first with the excited gla.s.s, and then bringing the rubbed wax, when the same effect takes place.

_Twelfth Experiment._

To show the important elementary truth, that in all cases of electrical excitation the two kinds of electricity are generated, take a dry roll of flannel, and holding it as lightly as possible, rub it against a bit of wax. If the flannel is brought to the electroscope, the leaves repel each other, and they immediately fall when the wax is now approached, because the flannel is in the positive or vitreous state of electricity, whilst the sealing-wax is in the negative or resinous condition.

_Thirteenth Experiment._

Any kind of friction generates electricity. A little roll of brimstone placed in a dry mortar and powdered, and then thrown on to the electroscope, quickly causes the repulsion of the leaves.

_Fourteenth Experiment._